Correlated Electromagnetic Levitation Actuator: A Reaction Sphere-Based Attitude Control System

To address problems experienced by current reaction wheels and control moment gyroscopebased
attitude control systems (ACS), researchers at National Aeronautics and Space Administration’s
(NASA’s) Marshall Space Flight Center (MSFC) have begun developing a reaction
sphere actuator based on correlated electromagnetic levitation that will be immune to destructive
bearing friction, momentum saturation, and gimbal lock. The Correlated Electromagnetic
Levitation Actuator (CELA) advances the state of the art of reaction sphere ACSs by employing
the concept of correlated magnetics. It is a frictionless, direct-drive reaction sphere that
harnesses a unique technology with an array of applications across multiple disciplines. Correlated
electromagnets function in a manner that is analogous to a matched filter; the convolution
of two signals is peaked at the index representing the greatest match. For CELA, the signals
are the patterns of magnetic flux density as a function of position. The magnitude of the convolution
equates to an attractive or repulsive force, and these forces can be azimuthal or radial.
The development of CELA is based in four distinct disciplines: Advanced Manufacturing,
Prototype Development, Electromagnetic Modeling, and Controls. We are developing novel
manufacturing techniques required to build arrays of permanent and electromagnet dipoles
on curved surfaces. To print the permanent magnetic array, we have developed a probe with
pyramidal magnets that will reside on a robotic arm to induce localized magnetic fields on
a surface. We have also used high temperature ovens to erase dipole patterns from a permanent
magnet by heating the surface to its Curie temperature. A number of test articles
and prototypes have been developed using additive manufacturing methods. These prototypes
have included hemispherical motors to test the drive algorithm, and a levitation test bed that
demonstrates a magnetic bearing method based on attractive magnetic forces and ratiometric
Hall effect sensors. We developed an array of electromagnetic dipoles on a printed circuit
board (PCB) with individual H-bridges controlling each coil. This device created various flux
density patterns and we measured their magnetic fields using a custom Hall effect 3-D probe
and a LabVIEW virtual instrument. These data will serve as a benchmark for characterizing
the accuracy of future models. Current work is focused on modeling the magnetic fields of our
prototype arrays using COMSOL finite element analysis (FEA) and verifying the model against
our test data. Accurate modeling will allow us to quickly test new patterns of electromagnets
and their macro behavior. Eventually, the magnetic field models will be implemented in our
controls simulations to facilitate precise control of the reaction sphere. Initial model results
agree with field measurements to within 1 G (5% of measured flux density). Currently, we
are testing different material properties of the electromagnets and their magnetic fields and
thermal effects. These results will be used to refine the design of the electromagnetic dipoles.
Our control efforts have centered on developing commutation, levitation, and field pattern
shaping hardware in the form of breadboards and PCBs with software running on a local microcontroller.
In addition, our partners developed MATLAB Simulink models to demonstrate
a PID controller that mitigates disturbance forces resulting from the interaction of drive and
levitation magnetics. Finally, we have designed a three-axis test stand that will be used in future
work to demonstrate CELA’s orientation control capability.